The present invention relates to a photosensitive resin plate for letterpress printing, which has a unique protective layer, which offers easy production of a printing plate, good handleability, and is water-developable.
The xe2x80x9cphotosensitive resin plate for letterpress printingxe2x80x9d referred to in the present invention (hereinafter referred to also as xe2x80x9cphotosensitive resin platexe2x80x9d) is a photosensitive resin composition molded into a thickness of ordinarily 0.5 to 10 mm, sandwiched between two film layers. In such a photosensitive resin plate, one film is peeled; the film-peeled side of the plate is photosensitized via a negative film; the non-photosensitized portion is removed; thereby, a relief is formed.
Formation of the relief (hereinafter referred to also as production of printing plate) is generally conducted by adhering a negative film to a photosensitive resin composition, applying an actinic ray via the negative film to selectively expose the composition to the actinic ray, and then removing the unexposed portion to form a relief.
In the relief formation, the surface of the photosensitive resin composition is sticky in many cases and usually invites various problems. For example, when a negative film is adhered onto the photosensitive resin composition and is later peeled, the peeling is insufficient due to the stickiness of the composition, damaging both the formed relief (printing plate) and the negative film; bubbles form at the time of adhesion of the negative film, making it impossible to achieve uniform adhesion; thus, the reproducibility of the image is low.
The methods adopted most widely for alleviating the above problems include a method which comprises covering the surface of a photosensitive resin composition with a thin protective layer removable with a developing solution. This method is specifically described in, for example, JP-A-51-49803 (a polyvinyl alcohol having a saponification value of 90% or more), JP-A-54-68224 (a water-soluble polymer) and JP-A-56-110941 (a soluble polyamide).
When there is used a photosensitive resin composition developable with an aqueous treating solution, generally a protective layer is used containing a water-soluble polymer (e.g. a polyvinyl alcohol or a water-soluble cellulose) so that the protective layer also is made developable with the aqueous treating solution.
When such a water-soluble polymer is used; however, the following occurs: the resulting photosensitive resin becomes substantially hydrophilic and therefore contains absorbed water; the photosensitive resin is covered with an ultraviolet-transmitting film and used in a vacuum state in order to enable, for light exposure, uniform adhesion of a negative film to the plate; the absorbed water in the photosensitive resin becomes a water vapor owing to the heat of light exposure and vaporizes from the photosensitive resin layer. As a result, there arises a problem that the water vapor softens the protective layer containing a water-soluble polymer, inducing excessive adhesion of the protective layer to the negative film. Further there arises a problem that the excessive adhesion makes it impossible to remove the water vapor, resulting in the formation of bubbles between the negative film and the plate. These problems arise particularly when a large printing plate is produced or after there has been a long exposure time in order to obtain a precise image. If part of the image to be reproduced happens to be at the site(s) where bubbles have been formed, the reproducibility of that part of the image is extremely low.
When the above-mentioned water-soluble polymer is used, there also takes place the following problem. During production of the printing plate, a cover film (ordinarily, PET film) used for protection of a protective layer is peeled, and the protective layer absorbs water and generates small wrinkles (hereinafter referred to also as wrinkles formed by water absorption) at the surface. When the resulting plate material having these small wrinkles is subjected to light exposure, the small wrinkles are reproduced in the relief. These wrinkles formed by water absorption are different in size depending upon the composition of the protective layer and have heights ranging from about 3 xcexcm to 20 xcexcm. This problem is a big obstacle to the spread of a water-developable photosensitive resin plate.
To suppress such excessive adhesion and wrinkles formed by water absorption, it was proposed to apply a matting treatment to a cover film to make matte the side of a protective layer which comes in contact with a negative film. This attempt, however, has a problem in that the matting treatment makes the total operational time long and incurs a higher cost and the control of the surface roughness in the matting treatment is very difficult; and also there is a problem in that the matting treatment causes light scattering and the reproduced image has a low resolution. There is a further problem that the matting treatment strengthens the adhesivity of protective film with cover film, reducing the peelability of cover film.
The present invention aims at providing a photosensitive resin plate for letterpress printing, which is developable with an aqueous treating solution, which can suppress excessive adhesion, wrinkles formed by water absorption and excessive adhesion, generated during production of printing plate, which enables excellent peelability, and which is superior in resolution.
The present inventor made an intensive study in order to solve the above problems. As a result, the present inventor found that when the photosensitive layer is protected by a protective layer having a multilayer structure comprising an intermediate layer and a surface layer, wherein the intermediate layer is in contact with the photosensitive layer and contains a water-soluble resin, and wherein the surface layer contains a resin having a water absorption of 5% by weight or less and having substantially no water-developability, that the surface layer surprisingly exhibits water-developability. The present invention has been completed based on the above finding.
The present invention is as follows.
(1) A water-developable photosensitive resin plate for letterpress printing which comprises a hydrophilic photosensitive resin layer and a protective film contacting therewith, wherein the protective film consists of at least two layers, of which one is a surface layer containing a resin having a water absorption of 5% by weight or less and the other is an intermediate layer containing a water-soluble resin which is in contact with the photosensitive resin layer.
(2) A photosensitive resin plate according to item 1, wherein the intermediate layer further contains fine particles of a hydrophobic resin having a water absorption of 5% by weight or less and the weight ratio of the hydrophobic resin fine particles and the water-soluble resin is 95/5 or less.
(3) A photosensitive resin plate according to item 1 or 2, wherein the intermediate layer and/or the surface layer contains an ultraviolet absorber.
The photosensitive resin plate for letterpress printing used in the present invention generally has a laminated structure consisting of a base film (ordinarily, a polyethylene terephthalate (PET) film) used as a substrate for maintaining dimensional stability, an adhesive layer, a photosensitive resin (composition) layer, a protective layer and a cover film (ordinarily, a PET film) in this order.
The protective layer is ordinarily formed on the back side of the cover film, and the side of the protective layer not in contact with the cover film is adhered to the photosensitive resin layer.
In the present invention, the protective layer consists of at least two layers and has a surface layer and an intermediate layer. The surface layer is in contact with the cover film, and the intermediate layer is in contact with the photosensitive resin layer.
The water-soluble resin used in the intermediate layer according to the present invention can be any resin which is water-dispersible or water-soluble. Specifically, the water-soluble resin can be any resin which is dispersible or soluble in an aqueous washing solution. For the resin, there can be mentioned, for example, polyvinyl alcohol, polyvinylpyrrolidone, water-soluble nylon, sodium polyacrylate, polyacrylamide, polyethylene oxide, polyethyleneimine and water-soluble cellulose derivative. Use of a polyvinyl alcohol is preferred in view of its stability when dissolved in water and the mechanical strength of the film formed therewith.
The fine particles of hydrophobic resin usable in the intermediate layer according to the present invention refer to particles which are composed of a resin having a water absorption of 5% by weight or less and which have particle diameters of 5 xcexcm or less. Larger particle diameters cause light scattering more easily; therefore, the particle diameters are preferably 2 xcexcm or less, more preferably 0.5 xcexcm or less.
As the hydrophobic resin, there can be mentioned, for example, acrylic resin, styrene-based resin, vinyl chloride-based resin, vinylidene chloride-based resin, polyolefin type resin, polyamide type resin, polyacetal type resin, polycarbonate type resin, polyester type resin, polyphenylene sulfide type resin, polysulfone type resin, polyetherketone type resin, polyimide type resin, fluororesin, silicon-based resin, urethane-based resin, urea-based resin, melamine-based resin, guanamine-based resin, epoxy-based resin and phenolic resin. These resins are used in the form of fine particles.
As the method for converting the resin into fine particles, there can be mentioned, for example, suspension or emulsion polymerization in which microgranulation takes place, and microgranulation of a resin by means of heat, solvent or the like. Microgranulation by emulsion polymerization is preferred from a standpoint that the fine particles obtained are stable in the presence of water.
The water absorption of the hydrophobic resin fine particles is preferably 5% by weight or less, more preferably 4% by weight or less because they are added in order to reduce the water absorption of the whole water-soluble resin mixture.
The water-soluble resin and the hydrophobic resin fine particles can be easily made into a thin film by mixing water as a main solvent with an alcohol or a water-soluble organic solvent, which is used to improve the wettability, drying rate, etc., to prepare a solution, mixing the resin and the fine particles into the solution, coating the resulting mixture on a film (e.g. a polyester film) in a film-state, followed by drying. The thin film obtained can have different flexibility and water developability by use of a plasticizer, a surfactant, etc.
When the intermediate layer in the present invention contains hydrophobic resin fine particles, the weight ratio of the hydrophobic resin fine particles and the water-soluble resin is 95/5 or less, preferably 90/10 to 5/95, more preferably 85/15 to 20/80. When the weight ratio is more than 95/5, it is difficult to obtain a water-developable film as a protective layer. When the content of the hydrophobic resin fine particles is low, it is difficult to suppress the formation of wrinkles caused by absorption of water, in peeling of cover film particularly in a highly humid environment.
The thickness of the intermediate layer is selected in a range of 0.1 to 20 xcexcm, preferably 1 to 10 xcexcm. When the thickness of the intermediate layer is smaller than 0.1 xcexcm, the water-developability of the whole protective layer is strikingly low. When the thickness of the intermediate layer exceeds 20 xcexcm, no further improvement is seen in the effect of the protective layer and the resolution of the relief is low.
The resin constituting the surface layer of the present invention can be any resin having a water absorption of 5% by weight or less and which can transmit an ultraviolet light. Practically, it can be a resin which can exhibit water-developability when used in combination with the intermediate layer. The water absorption is preferably 4% by weight or less, more preferably 3% by weight or less.
The content in the surface layer of the resin having a water absorption of 5% by weight or less is 30% by weight or more, preferably 50% by weight or more, more preferably 70% by weight or more, further preferably 90% by weight or more. When the content is less than 30% by weight, the surface layer has a low film contraction and expansion property and tends to form wrinkles even when the resulting photosensitive resin plate is deformed slightly during handling.
As the resin, there can be mentioned, for example, acrylic resin, styrene-based resin, vinyl chloride-based resin, vinylidene chloride-based resin, polyolefin type resin, polyamide type resin, polyacetal type resin, polycarbonate type resin, polyester type resin, polyphenylene sulfide type resin, polysulfone type resin, polyetherketone type resin, polyimide type resin, fluororesin, silicon-based resin, urethane-based resin, urea-based resin, melamine-based resin, guanamine-based resin, epoxy-based resin, phenolic resin, cellulose type resin and modified cellulose type resin. A modified cellulose type resin is preferred from the standpoints of water-developability, water absorption, resolution and peelability of cover film.
The thickness of the surface layer is in a range of 0.1 to 10 xcexcm, preferably 1 to 5 xcexcm. When the thickness of the surface layer is smaller than 0.1 xcexcm, there is seen a reduction in the preventability of adhesion or suppressability of formation of wrinkles by absorbed water. When the thickness exceeds 10 xcexcm, water-developability is strikingly reduced.
Resolution can be improved greatly by allowing the surface layer and/or the intermediate layer to contain an ultraviolet absorber. Allowing the intermediate layer to contain an ultraviolet absorber is preferred so that the color of the ultraviolet absorber is hardly transferred to a negative film. The addition amount of the ultraviolet absorber varies depending upon the absorption constant of the absorber and it is preferred to control the addition amount so that the ultraviolet light transmittance of film becomes 20 to 70%. When the addition amount is less than 20% by weight, no improvement in resolution is seen and the sensitivity is low. When the addition amount exceeds 70%, the improvement in resolution is low.
As the material for the cover film, there can be mentioned polyethylene terephthalate (PET), polyethylene, polypropylene, polystyrene and other materials. A polyethylene terephthalate (PET) is most suitable in view of the dimensional stability, heat resistance and mechanical properties.
The cover film has a thickness of ordinarily 50 to 250 xcexcm. Solutions of compositions constituting the protective layer according to the present invention are each coated thereon, followed by drying, so that the as-dried thickness may be ordinarily 1 to 10 xcexcm. In this case, to the solutions of the compositions may be added a surfactant, a defoaming agent, a leveling agent, a penetrating agent, etc. in order to improve the coatability of the solutions on the cover film. A corona treatment may be applied in order to improve the adhesivity between the surface layer and the intermediate layer. The thus-formed thin film formed on the cover film is adhered to a photosensitive resin composition, whereby a photosensitive resin plate for letterpress printing can be produced.
A photosensitive resin plate for letterpress printing can also be produced by laminating, on a base film, an adhesive layer, a photosensitive resin (composition) layer, a protective layer and a cover film. In such a photosensitive resin plate for letterpress printing can be used a protective film according to the present invention.
The water-developable photosensitive resin plate for letterpress printing referred to in the present invention means a resin plate which contains a photosensitive resin composition composed generally of a hydrophilic polymer, other polymer and additives such as plasticizer, crosslinkable monomer photoinitiator, stabilizer, sensitizer, surfactant, gelling agent and the like and developable with an aqueous treating solution and which can be used in letterpress printing. As specific examples of the resin plate, there can be mentioned a photosensitive resin plate for letterpress printing using a photosensitive resin composition composed of a sulfonic acid group-containing polyurethane resin as a hydrophilic polymer, a styrene-isoprene block copolymer as other polymer, and, as additives, polybutadiene diacrylate, aliphatic diacrylate, dioctyl fumarate, maleinized polybutadiene, N-ethyltoluenesulfonamide, 2,2-dimethoxyphenylacetophenone, 2,2-di-t-butyl-p-cresol and sodium laurylbenzenesulfonate; a photosensitive resin plate for letterpress printing composed of a photosensitive resin composition containing a phosphoric acid ester group-containing hydrophilic radical copolymer (Zeoflex (trade name), a product of Nippon Zeon Co., Ltd.); and a photosensitive resin plate for letterpress printing composed of a photosensitive resin containing crosslinked resin fine particles (FLEXCEED (trade name), a product of Nipon Paint Co., Ltd.).
As the aqueous treating solution (developing solution) used in production of printing plate, there can be used water or a solution obtained by adding, to water, as necessary a surfactant, a defoaming agent, a dispersing agent, an emulsifier, a corrosion inhibitor, an antiseptic, a pH-controlling agent, etc.